This invention relates to a switching circuit for connecting an AC source to a load, and particularly to a solid state AC relay circuit which triggers an active switch element such as a triac when the value of the AC voltage to be controlled is substantially zero.
Relay devices generally respond to an input signal and close a switch in a path of main current flow. Where the path of main current flow conducts alternating current between a source and a load, it is desirable to sense the incoming signal and to close the switch during or near the next succeeding passage of the source voltage through zero. This avoids the transient problems that might otherwise occur if the switch were closed at an instant when the AC voltage being controlled had a substantial value.
The most convenient switch available for turning alternating current on and off is a so-called triac, a thyristor that breaks down when a voltage is applied in either direction across the two main electrodes and a correspondingly poled voltage is connected to a gate or control electrode. Conduction continues until the voltage at the main electrodes is removed or reversed. In constrast to a silicon controlled rectifier, conduction may occur in either direction.
In order to switch on the triac at or near the zero passage of the voltage, complicated circuits which draw large current have been used in past devices to avoid applying the input signal until the zero passage time occurs in the next alternating current cycle. For example, the circuit disclosed in U.S. Pat. No. 3,648,075 utilizes two diode bridges, an SCR, four transistors, three capacitors, a transformer, and additional circuitry. Such an arrangement is comparatively complex and draws a substantial amount of current to energize these components.
An object of the invention is to avoid these difficulties.
Another object of the invention is to improve devices at this time.